Methods for producing corrosion resistant electrodeposited nickel coatings

ABSTRACT

Embodiments of the present methods deposit smooth, semi-bright nickel coatings from a nickel bath at room temperature, with relatively high concentrations (between about 5 and about 10%) of an organic modifier (such as butanol) under acidic conditions and using a modified pulse potential. The methods for electrodepositing nickel coatings result in nickel coatings that have improved internal structure and corrosion resistance.

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 62/347,938, entitled “Methods for Producing Corrosion ResistantElectrodeposited Nickel Coatings,” filed on Jun. 9, 2016, the entirecontent of which is hereby incorporated by reference.

BACKGROUND

This disclosure pertains to electrodeposited nickel coatings andparticularly to methods of producing electrodeposited nickel coatingshaving improved internal structure and corrosion resistance.

Corrosion inhibitive coatings are one of the most common ways used toprevent undesirable corrosion to the base substrates of many materials.Nickel coatings are widely used to create protective coatings for marineengineering, aeronautical, and other general decorative applications.Nickel is valued for providing strong adhesion to stainless steels, goodmechanical properties, and providing corrosion resistance for theunderlying steel. Nanocrystalline nickel coatings, which have increasedresistance to pitting corrosion, can be created by electrodeposition. Inrecent years, additional components (i.e. ceramics, metal oxides,polymers, etc.) have been added to nickel coatings to improve theprotection mechanism of the coatings in corrosive environments. Thesecomposite metal coatings have been studied for their improved physicaland corrosion resistant properties. The additional components chosen forinclusion into the coatings are often limited by their water solubilityor flocculation properties in the electrochemical baths. Due to this,synthetic routes such as casting, accumulative roll bonding, and hot dipmethods are often used, but at higher costs and often loss of controlover the morphology.

Recently, a number of new electrolyte systems have been developed tolower the cost and widen the pH range for nickel deposition; however,the most common still in use are Watts type baths, which take advantageof the complexing effects of borate ions and its ability to cathodicallyshift hydrogen evolution. Boric acid has a long history as an additivefor nickel electrodeposition and has been shown to provide a bufferingeffect at the electrode surface, decreasing the amount of hydrogenembrittlement in Ni coatings. Though the mechanism is not wellunderstood, boric acid forms a weak complex with nickel in solution thatis advantageous for deposition at low current densities, necessary fordepositing nanocrystalline nickel.

Addition of organic solvents can also have advantageous effects on themechanisms of crystal growth and morphology, but the resultant corrosioninhibition of these nickel coatings has not been tested. There are someinstances in literature of aqueous-organic systems used to deposit metalcoatings. Previously, N,N-dimethylformamide (DMF) was successfully usedas an organic modifier to control microstructure, current efficiency,and hardness for nickel electrodeposits. More recently, nickel basedalloys and nanocomposite coatings electrodeposited in pure DMF wereexamined as aprotic solvent baths to deposit metals that typically can'tundergo deposition in aqueous systems. Microcracking is often observedin these systems due to internal strain from the growth mechanism causedby large amounts of DMF.

Aliphatic alcohols have also been used as modifiers to generate finegrained nickel electrodeposits. The addition of unsaturated alcoholssuch as 2-butyne-1,4-diol and propargylic alcohol into a nickelelectroplating bath were shown to induce inhibited growth modes onnickel crystallization during electrodeposition but it wasn't shown tocontrol isolated or polycrystalline growth planes. The effects ofn-propyl, allylic, and propargylic alcohol were also studied and it wasfound that these aliphatic alcohols caused a cathodic shift in thereduction potential of nickel as well as an inhibitory effect on thecurrent efficiency in Watts type baths. It was seen that as unsaturationof the molecule increased, the reduction potential shifted morecathodically. It was also shown that both cis and trans 1,4-butenediolcompletely inhibited discharge of nickel above a concentration of 25 mM.Oxygen containing species, like these diols, containing unsaturatedbonds will adsorb parallel to the electrode surface to be hydrogenated,which is beneficial but potentially decreases the natural efficiency ofborate species for removing adsorbed hydrogen at the electrode surface.This adsorption affinity also limits the amount of additive that can beadded into the plating solution. Even though increasing unsaturationhelps dehydrogenate the electrode surface, the alcohol will stronglyadsorb and can shut down the reaction if the concentration is too high.Typically, no more than 50 mmol (˜0.05%) of the unsaturated alcoholadditive can be added to the plating solution.

Electrodeposition of composite materials for use as corrosion inhibitivecoatings containing nanoparticle size dopants is a growing field ofresearch, but so far, is limited to purely aqueous systems. Therefore,it is important to explore new organic additives for plating solutionsthat can produce a wide variety of composite coatings at lowtemperatures, with small grain size and low strain. This aqueous-organicbath system can help stabilize nanoparticles of very low solubilityduring the electrodeposition of nickel coatings.

SUMMARY

The present disclosure relates generally to methods forelectrodepositing nickel coatings that have improved internal structureand corrosion resistance. The present methods deposit smooth,semi-bright nickel coatings from a nickel bath at room temperature, withrelatively high concentrations (between about 5 and about 10% by volume)of an organic modifier (such as butanol) under acidic conditions andusing a modified pulse potential. Butanol (BuOH) has terminal alcoholsand no unsaturated bonds, with a nonpolar chain that can act as a pseudosurfactant, potentially stabilizing non-hydrophilic nanoparticles. Theimportant role borate plays in complexing with nickel and facilitatingthe reduction process can be inhibited by mono or polyunsaturated diolcompounds, known to complex with borate in solution.

A series of embodiments of nickel coatings deposited from bathscontaining BuOH demonstrated improved reduction potential, growthinhibition, strain, and corrosion resistance to chloride attack. Theexemplary coatings were nanocrystalline with particle sizes between16-35 nm as calculated from Williamson-Hall analysis. No cracking of thefilms was observed in the scanning electron microscopy images, even asthe percentage of butanol increased, which is typically observed inother aqueous-organic baths. The corrosion resistance of the nickelcoatings in a 3.5% sodium chloride solution was the best for coatingsdeposited from a plating solution containing 5 and 10% butanol. E_(corr)was shifted from −0.405 V for the nickel coating from the additive freebath to −0.234 V for the nickel coating from the 5% butanol solution.EIS results indicated a 100% improvement in the corrosion resistance forthe nickel coatings from the butanol solution. Higher percentages oforganic modifiers in the aqueous plating bath help open up the use ofnon-electroactive components to produce new composite coatings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a cyclic voltammetry study at a scan rate of 10 mV/s forbath solution samples containing no butanol and without nickel (A1α) andwith nickel (A1).

FIG. 1B shows a cyclic voltammetry study at a scan rate of 10 mV/s forbath solution samples containing 5% butanol and without nickel (B1α) andwith nickel (B1).

FIG. 1C shows a cyclic voltammetry study at a scan rate of 10 mV/s forbath solution samples containing 10% butanol and without nickel (B2α)and with nickel (B2).

FIG. 2 shows X-ray diffraction patterns of each nickel coating A1, B1,and B2.

FIG. 3A shows a Williamson-Hall plot for nickel coating A1.

FIG. 3B shows a Williamson-Hall plot for nickel coating B1.

FIG. 3C shows a Williamson-Hall plot for nickel coating B2.

FIG. 4 shows OCP versus time for each nickel coating immersed in 3.5%NaCl solution for 14 days.

FIG. 5 shows an equivalent circuit model for a nickel coating layer witha thin passive oxide layer displaying some micropores within thecoating.

FIG. 6 shows Nyquist plots and fittings for nickel coatings (a) A1, (b)B1, and (c) B2, using the circuit displayed in the inset. Black linesare the fitting curves and the dots represent the data from the EISexperiment.

FIG. 7 shows Bode phase angle plots for nickel coating samples. Blacklines are the fitting curves and the dots represent the data from theEIS experiment.

FIG. 8 shows potentiodynamic polarization curves for fresh coatings in3.5% NaCl.

FIG. 9 shows Tafel polarization curves for each coating in 3.5% NaClafter 14 days immersion in 3.5% NaCl.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Generally, the present disclosure relates to electrodeposited nickelcoatings having improved properties such as corrosion resistance. Inpreferred embodiments, the nickel coatings are electrodeposited using anaqueous organic bath containing about 5% to about 10% of an organicmodifier such as butanol. The aqueous organic bath preferably containssodium borate and is preferably adjusted to an acidic pH level,preferably having a pH of about 2.5 to about 3.5. The coatings displayminimal strain and nanocrystalline particle size using a pulse potentialdeposition at room temperature. After 14 days immersion in 3.5% NaCl,coatings that were deposited using an aqueous organic bath containing 5%or 10% BuOH had an improved resistance to corrosion showing ˜100%increase in the R_(ct) value (73672 Ωcm²) over a coating depositedwithout using the BuOH modifier. Linear polarization displayed an anodicshift of E_(corr) for coatings deposited using the organic modifier, dueto the development of a thicker passive oxide layer.

The present methods for electrodeposition of nickel coatings using anaqueous organic bath containing an organic modifier such as butanolproduce additional advantages as well. Applied overpotential is lowerthan other deposition methods to form coatings, needing only a 0.5-1.0A/dm² current density, which is 50 to 1000% lower than commercialall-sulfate plating baths used. This lower overpotential decreases theamount of hydrogen evolution that occurs during the plating process.Coatings are formed at ambient temperatures. Typical commercialtechniques commonly require temperatures of 40-70° C., thus no equipmentfor elevating and maintaining the heat of the bath is required. Butanoladdition has only minor effects on the conductivity of baths and doesn'tshut down the deposition process. Common additives such as1,4-butanediol, 1,4-butenediol, 1,4-butynediol, allylic alcohol, andpropargylic, are known to decrease the deposition process at additionsas high as 50 mM. Morphology of coatings show predominant (220)preferred orientation with only slight (5-15%) (111) and (311)character, with some controllability through the addition of butanol.Addition of butanol also decreases the grain size of nickel coatings,which decreases susceptibility to pitting corrosion. Addition of butanolalso does not increase internal strain within the coatings, whereasadditives tested before, i.e. DMF, have microcracking problems. Finally,passive oxide formation is thicker than coatings not formed in thepresence of butanol. Values for charge transfer resistance viaelectrochemical impedance spectroscopy testing were doubled by theaddition of butanol, compared to the modified all-sulfate bath withoutaddition of butanol.

Example 1. Electrodeposition of Nickel Coatings

Materials. All reagents were analytical grade and were used as receivedwith no further processing. Nickel coatings were electrodeposited fromsolutions containing 26.29 g/L NiSO₄.6H₂O (Alpha Aesar), 57.21 g/LNa₂B₄O₇.10H₂O (Fisher) and butanol (Mallinckrodt), adjusted to pH3.0±0.05 using 5.0 M H₂SO₄ (EM Scientific). By using the sodium borateand adjusting the pH to acidic levels, sodium cations can initiallyincrease conductivity of the solution and also maintain higherconductivity throughout the deposition, as Ni⁺² is deposited fromsolution. The solvent composition of the plating baths was designated asA1 (no BuOH addition), B1 (5% of BuOH added), and B2 (10% BuOH added).

Electrodeposition. An EG&G PAR potentiostat/galvanostat model 273A wasused for all depositions. Nitrogen was bubbled in all solutions priorand during electrodeposition. The electrode substrates were 10 mmstainless steel 430 disks (Ted Pella), attached to copper leads withconducting silver based epoxy and coated in resin epoxy to expose onlyone face. The stainless steel working electrodes were prepared bypolishing with 320 to 1200 grit SiC pads, followed by a final polishwith 0.30 μm alumina. Electrodes were submerged in 5 M H₂SO₄ for 120 s,rinsed with DI H₂O, and dried just prior to deposition and cyclicvoltammetry studies. Electrodeposition for all nickel coatings wascarried out by a modified pulse loop consisting of −1.08 V for 10 sfollowed by −0.6 V for 4 s, scanning at 100 mV/s between steps, until atotal charge of ˜40.0 coulombs was reached. The resulting coatings were˜6 μm thick for all these experimental parameters.

Reduction potential of Ni and the effect of BuOH. A Thermo Orion 550Aconductivity meter with a Thermo Orion 013005A platinum black four cellconductivity probe was used to measure the conductivity of the platingbaths. Cyclic voltammetry (CV), open circuit potential immersion testing(OCP), Tafel polarization, and electrochemical impedance spectroscopy(EIS) studies were performed with a PAR Parstat 4000 potentiostat(Ametek). Cyclic voltammetry of the solutions was carried out in a threeelectrode cell system, composed of a stainless steel (SS) workingelectrode, chromel counter electrode, and a saturated calomel (SCE)reference electrode. Potentiodynamic polarization, EIS, and OCP were allperformed in a solution of 3.5% sodium chloride diluted with deionizedwater. The EIS was scanned from 10⁵ Hz to 25 mHz with a perturbationamplitude of 10 mV. Potentiodynamic polarization scans were run from±250 mV at a scan rate of 1 mV/s.

Cyclic voltammetry was performed for each of the plating bath systems todetermine the effects of BuOH on the reduction potential at pH 3±0.05.Each solution was scanned between 0.5 V and −2.0 V, starting from OCP,at a scan rate of 10 mV/s. The reduction and oxidation potentials ofeach bath are presented in Table 1 below.

TABLE 1 E_(pc), E_(pa), i_(pc), and i_(pa) for each sample measured fromcyclic voltammetry at a scan rate of 10 mV/s. Sample E_(pc) (V) i_(pc)(mA) E_(pa) (V) i_(pa) (mA) A1 −1.22 18.4 −0.300 0.484 B1 −1.33 15.0−0.300 0.465 B2 −1.27 14.7 −0.287 0.853

Cyclic voltammograms for each sample bath are displayed with and withoutthe presence of nickel in FIG. 1. Electrolyte bath systems containing nonickel are shown with a designations and were performed to subtract anycapacitive effects of BuOH on the scan. The addition of BuOH in B1 (5%)and B2 (10%) shifted the reduction potential of nickel to slightly morecathodic potentials around −1.25 V. This is consistent with findingsthat aliphatic alcohols do cause a cathodic shift in the reductionpotential of nickel. The current slightly decreased as BuOH percentageincreased, due to a larger amount of adsorption onto the electrodesurface, limiting the active area for nucleation. Hydrogen evolution inthe cathodic region was considerable but expected at such large negativepotentials, but the characteristic suppression, due to the presence ofborate, was evident in this system. There was a small anodic peak on thereverse scan at ˜−0.30 V, which was due to the nickel metal oxidizing tonickel oxide on the surface. Anodic peak current, i_(pa), in allsolutions was minimal, and the organic modifier appeared to have littleeffect on the anodic potential, E_(pa), and i_(pa) in that direction.

Deposition conditions. A modified pulse reverse deposition provides moreuniform coatings of nickel and the upper and lower potentials werechosen after analyzing CV studies. An upper deposition potential of−1.08 V was selected, to minimize hydrogen evolution at the electrodesurface but still provide a large enough overpotential to deposit nickelfrom solution. Deposition of A1 (no organic modifier) was used as abenchmark to test the deposition quality for cathodic potentials between−1.0 V to −1.2 V. It was found that −1.08 V produced good quality anduniform nickel coatings with minimal hydrogen evolution. The lowerpotential of −0.6 V was determined experimentally and selected as thepotential where zero current flow was observed. A running scan rate of100 mV/s between these potentials was selected because it produced moreuniform deposits versus a square wave potential step method. The pulsedeposition was cycled until ˜40 C of charge was obtained to provide fullcoverage of the substrate. The thickness of the coatings was measuredusing a profilometer and averaged 6.30±0.11 μm for this amount ofcharge. The coating thicknesses did not vary with the addition ofbutanol to the solution. Even though the conductivity of the platingsolution was slightly lowered with the organic addition, it was stillhigh enough to allow good plating of the nickel; A1=26.53±0.57,B1=23.01±0.64, and B2=21.46±0.57 mS/cm. The appearance of all coatings(with and without butanol) was similar and semi-bright in appearance.

Example 2. Characterization

Techniques. Coating morphology was characterized by an Environmental FEIQuanta 200 scanning electron microscope. Powder x-ray diffraction wasused to measure crystallinity and composition of the coatings using aSeimens D-500 X-ray diffractometer. Scans were run from 30-100 degrees2θ at a 1 sec dwell time and 0.05° step size, using CuK, radiation withthe tube set to 35 kV and 24 mA. Coating thickness was measured using aVeeco Dektak 8 stylus profilometer.

X-ray diffraction (XRD). The effects of the addition of BuOH on thecrystal orientation of nickel were investigated with powder XRD and theresults are shown in FIG. 2. There are two predominate peaks observedfor each nickel coating, the (111) and (220) reflections. Thoseorientations are characteristic for face centered cubic (fcc) nickel,although the XRD patterns showed preferred orientation (220) for thecoatings. This mixed phase is a result of a lower reduction potential of−1.08 V applied during the deposition of the coatings. It has been foundthat a lower overpotential can cause a preference for [110] growthplanes when there is low current density, or in the presence of wettingor brightening agents. Samples B1 and B2, containing butanol, alsodisplay (220) preferred growth but slightly more (111) character, againconsistent with the observed cathodic reduction potential shift ofnickel. To better understand the influence of how organic additioninfluences the preferred orientation of the coatings, the relative peakintensities were analyzed by determining the relative texturecoefficients of the peaks ((111), (200), (220), and (311)) present inthe XRD patterns.

Additionally, due to the polycrystalline nature of the coatings,determining which orientation is more beneficial for corrosionresistance was of interest as well. The influence of each can berepresented by calculating the relative texture coefficient (RTC_(hkl))for each orientation and comparing it with the corrosion resistance ofthe films. Eq. [1] shows how this was performed,

$\begin{matrix}{{RTC}_{hkl} = {\frac{I_{p,{hkl}}/{I{^\circ}}_{p,{hkl}}}{\Sigma \; {I_{p,{hkl}}/{I{^\circ}}_{p,{hkl}}}}*100\%}} & \lbrack 1\rbrack\end{matrix}$

where I_(p,hkl) is the intensity of the peak for each sample at (111),(200), (220), and (311) reflections and I^(o) _(p,hkl) is the intensityof those reflections for the nickel standard reference(PDF#00-004-0850). The standard provides a random orientation pattern tocompare against any preferred orientation found in the samples. Resultsare listed in Table 2 below.

TABLE 2 Relative texture coefficients for (111), (200), (220), and (311)growth planes for each sample as measured with XRD. Sample RTC₁₁₁ RTC₂₀₀RTC₂₂₀ RTC₃₁₁ A1 5.26 4.85 72.32 17.66 B1 8.09 4.32 71.84 15.74 B2 7.804.27 72.66 15.26

Sample A1 has a relative texture coefficient of 72.32 for thepredominant (220) reflection, 5.26 for (111), 4.85 for (200), and 17.66for (311). Samples B1 and B2 display RTC values for (220) similar tothat of A1, at 71.84 and 72.66, respectively, which is also consistentwith the cathodic shift in nickel reduction potential. There is anoticeable increase in (111) and decrease in (311) coefficients for B1and B2.

Organic addition into the coating bath can lead to microcracking andinternal strain in nickel electrodeposits, so Williamson-Hall analysiswas performed. A correction for instrumental broadening was done using aSi powder 325 mesh (Alpha Aesar) sample which matched thePDF#00-027-1402 file. The particle size and strain were calculated bysolving for the full width at half maximum (FWHM) of the nickel peaksfor reflections (111), (220), and (311) using Eq. [2],

B _(r) ² =B _(o) ² −B _(i) ²  [2]

where B_(r) equals the corrected FWHM of each peak, B_(o) is theobserved FWHM of the nickel film peaks, and B_(i) is the instrumentalbroadening value calculated from a 325 mesh silicon powder standard.Lattice strain and particle size (shown in Table 3 below) was found byplotting B_(r) cos θ vs. sin θ (FIG. 3) for the corrected nickel peaks.The strain for each coating was minimal, most likely due to the presenceof borate and absence of allylic groups on the organic modifier. The lowstrain values are promising results for these nickel coatings depositedusing the aqueous/organic baths. Particle size was in the nanometerrange from ˜16 to 35 nm, which is predicted from the addition of aborate electrolyte, the addition of organic modifier, and a pulsereverse deposition.

TABLE 3 Strain and particle size of each sample as determined fromWilliamson-Hall analysis of the x-ray diffraction data Sample Strain (η)Particle Size (nm) A1 0.0083 16.09 B1 0.0043 19.05 B2 0.0175 34.48

Surface morphology. All samples were examined with scanning electronmicroscopy (SEM) after electrodeposition. For the freshly depositedsamples, a uniform coating across the surface with smooth fine graineddeposit and no microcracking was observed for the nickel coating, whichmatches the minimal strain present in the deposits as indicated by XRD.All samples were nanocrystalline, as determined with XRD, and similar inmorphology. The presence of (220) geometry appears as pyramidalstructures, but due to the mixed (111) and (220) orientations, thesurface is comprised of a hybrid geometrical appearance for all thesamples. Samples B1 and B2 appear to have smaller grains than that ofA1.

Example 3. Corrosion Studies

Open Circuit Potential (OCP) Immersion Studies. After electrodeposition,the coatings were submerged in a 3.5% NaCl solution to simulatecorrosion and the OCP was monitored for 14 days prior to Tafelpolarization and EIS analysis. The immersion study results can be seenin FIG. 4. All samples showed anodic shifts in OCP, due to the formationof an oxide layer, providing barrier protection, and evidence ofgeneralized corrosion on the surface was observed for all samples.Sample B2 maintained the most anodic OCP at −0.252 V. After 14 days,each sample was analyzed by linear polarization to determine E_(corr)and i_(corr) values. Each samples was immersed in fresh NaCl (3.5%) for1 hour, until a stable OCP was reached prior to analysis.

Electrochemical impedance spectroscopy. Electrochemical impedance (EIS)was run to better understand how BuOH addition affects the corrosionresistance of the electrodeposited nickel coatings after a 14 day soak.ZView software was used to model the EIS results and calculate theequivalent circuit values of each circuit element. Each sample wasnormalized to a 1.0 cm² area and modeled successfully. The equivalentcircuit model shown in the inset FIG. 5, contains 5 elements; 3resistors and 2 constant phase elements, which model non-idealcapacitors. The circuit models a nickel coating layer with a thinpassive oxide layer displaying some micropores within the coating. Theoxide layer will have formed after the immersion time, but would be thincompared to the nickel coating. For this circuit, R_(s) represents theresistance of the 3.5% NaCl solution, CPE_(f) represents the capacitanceof the passive coating where α₁ is a coefficient between 0 and 1, 1being ideal, representing the behavioral deviation from an idealcapacitor. R_(p) represents the resistance of the electrochemicalreaction between the solution and the nickel coating, both on thesurface and within the micropores. CPE_(dl) and α₂ represent the doublelayer capacitance between the electrolyte and the coating interface atthe bottom of the pores. R_(ct) is the charge transfer resistance, whichis related to the corrosion resistance. This circuit model matches othermeasurements for nickel coatings on steels. The calculated values forthe circuit elements are listed in Table 4 below.

TABLE 4 Calculated circuit elements generated using ZView software fromthe EIS data. CPE_(f)· CPE_(dl) Sample R_(s) (Ω) (μFcm⁻²) α₁ R_(p)(Ωcm²) (μFcm⁻²) α₂ R_(ct) (Ωcm²) A1 12.99 75.3 0.826 945 81.6 0.62532653 B1 10.13 73.2 0.813 1662 50.8 0.718 73672 B2 10.58 49.3 0.869 165638.3 0.610 69290

The Nyquist and Bode plots for EIS are shown in FIGS. 6 and 7,respectively. Nyquist plots and fittings for each sample show a distinctsingle semicircle pattern (FIG. 6). Values for R_(s) range between 10 to13Ω, shown in Table 4, displaying a reliable fit for each sample. R_(p)increases slightly for B1 and B2 over A1, indicating an increase in thepassive film resistance. B1 and B2 display larger capacitive loopdiameters, giving larger R_(ct) values compared to A1, and a higherresistance to corrosion. This increase is most likely due to the smallergrain sizes of the deposits. Sample B1 displays the highest R_(ct) valueat 73,672 Ωcm², nearly double the value for A1. Increasing thepercentage of BuOH to 10%, in B2, leads to a slight drop in R_(ct), butonly negligible decreases in R_(p). It is apparent that when butanol ispresent during deposition, it modifies the deposition of nickel,possibly by stabilizing the NiOH colloid, slowing the rate of depositionand decreasing the grain size of the deposited nickel. Due to this,corrosion is slower, and the passive layer forms more quickly for thesefilms.

Bode phase angle plots (FIG. 7) were studied to determine how the phaseangle values model the capacitive effects and corrosion resistantbehavior of the coatings. All coatings tested displayed a broad phaseangle maximum, characteristic of the two time constants, which isconsistent with the circuit model and fitting. This suggests that thereare two capacitive responses, first for the interface of the nickelcoating to the substrate and the other represents the response due todiffusion of solution through an outer passive-oxide layer that isformed. This is displayed as a second maxima in the lower frequencyregions for all samples. All coatings approach 0° in the high frequencyregion, and the phase angle maximum occurred between 30-50 Hz, for everycoating except for sample B1, which occurred at 2 Hz. In the lowerfrequency range, sample A1 shows the lowest value, followed by B2 andB1. B1 sustains a higher phase angle value through the lower regions,indicating the most capacitive behavior, indicative of a smallerparticle size and surface charge. B1 and B2 phase angles are closer to90° than A1, indicating a more stable passive film.

Potentiodynamic polarization measurements. Potentiodynamic polarizationscans from −0.25 V to 1.0 V were run on fresh coatings in order tomeasure their resistance to corrosion in the active region of the scan,where the dominant reaction is oxidation of nickel (FIG. 8). The passiveregion formed for the fresh coatings is minimal compared to the soakedcoatings, but allows a measure of how thick a passive layer can beformed for the different coatings. Fresh coatings deposited from thesolutions containing BuOH, had rest potentials (E_(corr)) cathodic toA1, and a higher β_(a) slope, indicating an ability to form an oxidelayer more readily. The breakdown potential (E_(bd)) was located afterthe passive region, where there is a large onset of current, indicatingfailure of the coating due to pitting or crevice corrosion reaching thesteel substrate. By measuring the difference between the rest potentialand breakdown potential, the resistance of the passive layer tobreakdown can be measured for each coating. Samples B1 and B2 displayedthe largest difference in the active region (100 and 50 mV wider thanA1, respectively), showing that the smaller grain size helps with theonset of oxide layer formation more quickly. Addition of BuOH in theplating bath improved the initial oxide layer formation for the nickelcoating, and fits the EIS results.

Linear polarization was carried out for the coatings after soaking for14 days in a 3.5% NaCl solution. Coatings were immersed in a fresh 3.5%NaCl solution and scanned ±250 mV from OCP in both directions at a scanrate of 1 mV/s. The E_(corr) and estimated i_(corr) values were measuredby locating the intersection point of the extrapolated linear sectionsof the anodic (β_(a)) and cathodic (β_(c)) portions of the scan. Theresults are listed in Table 5 below and shown in FIG. 9 for eachcoating.

TABLE 5 E_(corr) and i_(corr) corrosion values for the nickel coatingscalculated from linear polarization measurements. Sample E_(corr) (V vs.SCE) i_(corr) (Acm⁻²) β_(a) (V/dec) β_(c) (V/dec) A1 −0.405 1.99 × 10⁻⁷0.185 0.393 B1 −0.234 3.35 × 10⁻⁷ 0.190 0.195 B2 −0.343 2.14 × 10⁻⁷0.241 0.191

All sample sets containing the organic modifier (B) displayed betterpassivation than sample A1, which can be seen by the slope of β_(a) andthe E_(corr) values for B1 and B2 were shifted anodically from the A1values. The slope of this linear region represents a passivity due to athicker oxide layer. The i_(corr) values were all in the same range of10⁻⁷ Acm⁻², with A1 displaying the lowest corrosion current of ˜2×10⁻⁷Acm⁻². The anodic slopes of β_(a), describe the onset of passivation andthe rate at which a passive oxide layer forms. The corrosion studiesshow again that there is an improvement in corrosion resistance whenadditions of BuOH are added to the plating bath.

What is claimed is:
 1. A method for electrodepositing nickel coatings,comprising: preparing an aqueous bath comprising nickel and about 5% toabout 10% by volume of an organic modifier, wherein the aqueous bath hasan acidic pH; submerging electrode substrates in the aqueous bath; andusing modified pulse reverse deposition to electrodeposit the nickelcoatings on the substrates.
 2. The method of claim 1, wherein theorganic modifier is butanol.
 3. The method of claim 1, wherein theacidic pH is about 2.5 to about 3.5.
 4. The method of claim 1, whereinthe aqueous bath further comprises sodium borate.
 5. The method of claim1, wherein all steps are carried out at room temperature.
 6. Anelectrodeposited nickel coating prepared by the method of claim
 1. 7. Amethod for electrodepositing nickel coatings, comprising: preparing anaqueous bath comprising nickel, sodium borate, and about 5% to about 10%by volume of butanol, wherein the aqueous bath has pH of about 2.5 toabout 3.5; submerging electrode substrates in the aqueous bath; andusing modified pulse reverse deposition to electrodeposit the nickelcoatings on the substrates, wherein all steps are carried out at roomtemperature.
 8. An electrodeposited nickel coating prepared by themethod of claim 7.